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An aerial view of the NELHA facilities in Kona, Hawaii. Makai has installed undersea pipelines that descend to 915 m (3,002 ft) water depth at a temperature of 4° C (39° F).

Seawater Air Conditioning (SWAC) takes advantage of available deep cold water from the ocean, a river, or lake, to replace conventional AC systems. Such a system can also utilize cold lake or river water as the cold source.

SWAC feasibility studies for a variety of sites indicate that electrical consumption is typically reduced by 80 to 90 percent. Simple payback can be from three to seven years, and long term costs can be half that of a conventional air conditioning system.

Introduction

Conventional AC

Conventional air conditioning is expensive to operate due to large electrical power requirements. The process uses evaporative cooling of a gas to transfer heat. A simplified illustration of the process is below:

An AC powered compressor compresses gas resulting in the generation of heat.

The gas (shown as red in the figure) runs through a set of coils for heat dissipation and condenses into a liquid.

The liquid passes through an expansion valve quickly evaporating into cold low-pressure gas.

The cold gas (shown as blue in the figure) runs through a set of coils for heat absorption cooling down the air inside a building.

Large commercial buildings use the expansion of the refrigerant gas to cool water instead of cooling the air directly (called a chiller unit). Chilled water is then pumped throughout the building to provide air conditioning. Furthermore, large systems utilize cooling towers to more efficiently cool the high-pressure side of the system (shown as red in the figure above and the top part of the figure to the left). All of these components, compressors, water pumps and cooling towers, contribute to the high operating expense of conventional AC systems.

Deep Sea/Lake Water AC

Along many ocean coastlines and lake shorelines, there is reasonable access to naturally cold water that is as cold or colder than the water used in conventional air conditioning systems. If this water can be tapped, then the significant power for operating mechanical chillers can be eliminated. The process is very similar to using chillers in conventional AC systems. The only difference is that the cold temperature is not achieved by evaporation of a liquid into a gas. Rather, it is retrieved from a natural cold water source – from a deep ocean or lake.

Basic Process

Water is pumped from a deep cold water source (ocean or lake).

The water is passed through a heat exchanger.

A closed-loop fresh water distribution system is pumped through the heat exchanger cooling the water.

The cooled water is distributed to buildings for air conditioning.

Main Components

There exist three main components of deep water cooling. These basic components can be optimized for each specific location, climate and building.

A sea/lake water open loop supply system which pumps deep cold water through a heat exchanger and returns the warm water through a shallow outfall (note that in Enwave’s design the water is not returned to the lake but used for the city’s water supply).

A fresh water closed loop system pumps warm water through the cooling station heat exchanger and distributes the cooled water among commercial, residential and institutions for air conditioning.

A heat exchanger (cooling station) transfers heat from the fresh water distribution loop resulting in cold water for air conditioning purposes.

Auxiliary Chiller

In some cases, it may be either too costly or impractical to supply seawater at the necessary low temperatures to maintain minimum temperatures in the chilled water loop. The distance offshore to reach sufficiently cold water might be prohibitive or the ocean depth may simply not be available. It is sometimes economically possible to use auxiliary chillers to supplement the cooling provided by the seawater exposure. This is illustrated to the left. The fresh chilled water is first cooled by seawater through a heat exchanger and then secondarily cooled with an auxiliary chiller. The auxiliary chiller is basically a refrigeration system with its condenser cooled by the returning flow of cool seawater. With the condenser kept cool, the auxiliary chiller can operate at an extremely high efficiency – as high as double that of a conventional chiller.

Seawater Cooled Chiller

If a site with high air conditioning costs has no access to cold seawater, near shore seawater can also be used to reduce air conditioning costs. This is done by replacing an air-cooled or evaporative cooling tower cooled condenser with seawater cooling. If the condensing temperature of a conventional chiller unit can be reduced, energy savings will result. The amount of energy saved will depend upon the change in condensing temperature. Shallow surface seawater will typically be much cooler than the air, especially during the hottest time of year. Thus, an air-cooled condenser can be replaced by seawater cooling and obtain 25% or more energy savings. If the chiller uses an evaporative cooling tower for condenser cooling, usually shallow seawater can both improve chiller performance and can eliminate the noise, water demand and sewage fees associated with the evaporative cooling tower. The figure on the right shows a schematic of a conventional chiller with its condenser cooled with shallow surface seawater.

Cold Storage

A SWAC system has a high capital cost and a low operating cost. The peak capacity of the system must match the peak demand of the buildings that it serves. These demands are not constant throughout the day or throughout the year, and the total system is frequently not being used to its maximum capacity. Therefore, capital dollars are spent on a system that may not always be used to its maximum potential. A means of minimizing the capital cost is to use cold-water storage. The seawater air conditioning system would be operated 100 percent of the time and when the building demands are low, the excess capacity is directed into a storage system of cold fresh water. When A/C demand is at its peak, the cold water is drained from its storage to meet the demand. Cold water storage tanks are commercially available that are constant volume; the warm water remains at the top and the coldest water remains at the bottom. These tanks are now used in conjunction with conventional A/C systems to take advantage of low, off-peak electrical rates.

Environmental Benefits

Sea water air conditioning has many environmental advantages over that of conventional a/c systems:

90% reduction in energy consumption

reduction in the local energy grid requirements

decreased reliance on fossil fuels

reduced air pollution

reduced acid rain

reduced impact on global warming

reduced ecological & political impacts of resource extraction

no use of ozone-depleting chlorofluorocarbons (CFCs)

US federal air quality legislation has banned the manufacture and importation of these refrigerants

The existence of the deep water ocean heat sink results from natural climatic processes where water is cooled at the poles, becomes dense and sinks to deeper water. The figure at right illustrates a temperature profile in the tropics typical for the world’s deep oceans. 7°C or colder can be reached at 700m depth, 5°C or colder at 1000m. The deep-water portion of this profile changes little seasonally and therefore cold water is available on a year round basis.

There are significant secondary applications for this seawater. Secondary cooling, aquaculture, desalination and even agriculture can benefit from the cold seawater. Aquaculturists value the water because it is clean and disease free. When used in conjunction with a warm source of water, they can have any temperature seawater their product needs. Secondary cooling can be used in greenhouses and other locations where humidity control is not a major factor. Finally, research in Hawaii has shown that even an arid land can be made highly productive with low fresh water consumption by cooling the soil and the roots (resulting in condensation) of many tropical and non-tropical plants. Deep seawater is also desalinated and sold as a premium drinking water in Asia. The figure above is an aerial photo of the Natural Energy Laboratory of Hawaii Authority (NELHA) where many of these secondary applications are being used and researched.

Economic Viability

The economic viability of a SWAC system is site specific. Each location has unique opportunities as well as problems. The main factors influencing the economic viability of a specific location include:

The distance offshore to cold water: shorter pipelines are more economical than long pipelines.

The size of the air conditioning load: there is an economy of scale associated with SWAC – systems less than 1000 tons are more difficult to justify economically.

The percent utilization of the air conditioning system: The higher the utilization throughout the year, the higher the direct benefits.

The local cost of electricity: A high cost of electricity makes conventional AC more costly and SWAC, in comparison, more attractive. Any cost analysis should include current and future costs of electricity.

The complexity of the distribution system on shore: SWAC works best with a district cooling arrangement, where many buildings are cooled taking advantage of the economy of scale. SWAC is even more economical if this distribution system is compact.

The figure to the right illustrates the difference in lifetime costs for a conventional AC system and a typical SWAC system. The costs are broken down into capital, operating (energy) and maintenance. The primary cost of a SWAC system is in the initial capital cost. The operating and maintenance costs are small. For a conventional AC system, the primary cost is in the power consumed over its lifetime. Hence, SWAC systems are ideal for base load AC that has high utilization and conventional AC may be better for situations of infrequent use.

It’s important to note that there is a dramatic economy of scale as the size of the pipeline increases. The reason is that the cold water pipe costs per liter of water delivered decreases as the pipeline size increases and temperature rise via large pipelines is practically negligible. The figure to the left illustrates five SWAC scenarios of varying overall size; the two bars compare the life time cost difference between conventional AC and SWAC.

SWAC feasibility studies for a variety of sites indicate that electrical consumption is typcially reduced by 80 to 90 percent. Simple payback can be from three to seven years, and long term costs can be half that of a conventional air conditioning system. Not all locations, however, are ideal. Some have poor access to deep cold-water sources or the overall size is too small to be economical.

Makai’s SWAC model (METHOD™)

For over 25 years, Makai has continuously developed custom software for modeling the hydraulic and thermal aspects of fluid networks, and recently underwent a major overhaul with the help of a U.S. Navy research and development grant. Notably, the cost algorithms were upgraded and now account for 160 various costs applied across a dozen of the key construction steps for district cooling systems. This software have been used to model, analyze, and design district cooling networks, and especially SWAC district cooling systems. The model, called the Makai Economic, Thermal, and Hydraulic Optimization and Design software, or METHOD™ software, takes into account all of the major capital and operational costs for both systems and the complex interplay between the sub-system designs and operational costs. This enables an “apples-to-apples” economic comparison of district cooling versus an equivalent conventional A/C system. Other financial metrics, such as payback period and rate of return of the district cooling system, are also computed. The METHOD™ software consists of two main components: an engineering and an economic model. It considers the primary engineering and economic parameters associated with a particular SWAC site, produces a conceptual design, and provides a fair comparison of the cost of cooling provided by SWAC versus conventional air conditioning.

In order to reduce the costs of a district cooling system, METHOD™ is used to design and optimize components to minimize the overall levelized cost of cooling. The software is particularly useful for providing quick and cost-effective “what if” analyses to help the developer decide between possible design variations early in the project, such as evaluating whether or not to add a nearby A/C customer to the network. Users can instantly see the effect on levelized cost due to a change in the network. In the case of a SWAC system, METHOD™ includes accurate costs for the offshore seawater pipes that are derived from real construction projects – these are necessary to get an accurate project cost, and are something only a firm with significant offshore pipeline construction experience can offer. More than 25 years ago, Makai created the original SWAC model, and our engineers have been improving its functionality ever since.

The above screenshot shows the model being used to design another SWAC district cooling system in Mauritius for Urban Cooling. Once a layout is defined by the user, the major system components (deep seawater pipes, chilled freshwater network, etc.) and operations (pumping power, maintenance, etc.) are then optimized for a minimum cost of cooling. The Caribbean study suggests that district cooling is a clear economic and environmental winner versus conventional A/C.

Engineering model: Starting from a few client-provided inputs, this software determines a very basic conceptual design for a SWAC system that includes the following components:

Offshore pipes (intake and return water)

Pipeline Shore Crossing

Seawater Pumping/Heat Exchanger Station

Onshore Chilled Water Pump Station

Onshore Chilled Water Distribution System

The software defines the parameters (e.g. sizes, lengths, flow rates, power requirements, etc) for each of these major components.Economic model: Once an initial conceptual design is complete, Makai uses the software to assign a cost for the design, construction, operation and maintenance to each component in the SWAC system. The model then runs an optimization algorithm to minimize for the levelized cost of cooling. The optimized design then produces a cost estimate that allows a fair economic comparison between SWAC and conventional air conditioning, using a levelized cost analysis. Usually a SWAC system must look significantly better than a conventional AC system solution before Makai will recommend it.

Makai’s economic model is based on an analytical procedure developed by the Electric Power Research Institute (EPRI) in their Technical Assessment Guide [TAG]. The TAG model is an economic analysis method of comparing two alternate energy systems with different capital and operating costs.

One of the most important inputs to the economic model is the cost of the large and unique offshore pipelines. Makai’s strength is our extensive experience and knowledge of costs associated with constructing and installing large marine pipelines, as will be discussed below. No other firm has such an intimate knowledge of the costs of these unique submarine pipelines.

Project Experience

The key cost and risk component of any SWAC system is the offshore pipeline. The lack of a low-cost methodology for the installation of these pipelines prevented SWAC development in the 1970’s and 80’s. Today, the technology for the successful installation of large diameter (>2 meter) pipelines to depths of 1,000 meters and greater is available. Numerous successful deep seawater intake pipelines have been designed by Makai that were installed and operated successfully – not a single one has failed due to a design flaw. Many of these pipelines have been for Seawater Air Conditioning (SWAC) systems. Click here to see a summary of some of Makai’s experience in pipeline design, analysis and deployment.

Contact Makai to discuss your SWAC Project

Makai has been working on SWAC projects consistently for more than 20 years. Our database of worldwide site information and experience with successful SWAC projects enables us to assess a site rapidly for its viability. For those interested in developing SWAC systems, Makai provides a preliminary opinion of viability free of charge. If the site looks promising, Makai will propose one or more options for a SWAC feasibility study for your particular site that fits within your budget.